The human body has various types of vessels for the exchange of fluids. Such vessels include bronchi, which carry air in lung tissue; bile ducts, which carry bile in liver tissue; lymph vessels, which carry lymph throughout the body; and blood vessels, which carry blood throughout the body. Denaturing of collagen present in vessel walls plays a major role in vessel sealing. Because all these types of vessels have a similar collagen structure in the walls, the processes involved in their sealing are similar. Blood vessels require sealing in virtually any surgery, and the following discussion, while applying to all above-mentioned vessels, is exemplified by sealing of human blood vessels.
There are several types of blood vessels, including arteries, veins, and capillaries. Arteries carry oxygen-rich blood away from the heart. As a result of heart contractions, arteries are under pressure typically varying from 80 mmHg to 120 mmHg. In contrast, veins carry oxygen deprived blood towards the heart and have a constant blood pressure which is typically below 10 mmHg. Capillaries are the small blood vessels through which the actual exchange of water and chemicals between blood and tissue occur. Their diameter is large enough to allow red blood cells to pass single file. In tissues, capillaries typically form dense networks referred to as capillary beds. Vessels of each type may also vary significantly in size and flow rate. Generally, however, the vessels of larger diameter have a higher flow rate.
During surgery, tissue dissection results in severing vessels that pass through the plane of dissection. It is desirable to seal the severed vessels quickly to prevent the escape of vital fluids and to reduce the risk of foreign matter entering into the vessel. Bleeding from severed blood vessels during surgery is one of the most well studied and often encountered situations requiring vessel sealing. Severing capillaries results in tissue bleeding. Severing of larger vessels may result in excessive bleeding, which obscures the surgical site, may require blood transfusions, and even threaten the patient's life. Therefore, the rapid coagulation of tissues and sealing of blood vessels is desirable. Similarly, the rapid sealing of lymph vessels, bronchi, and bile ducts when severing of those vessels occurs is also desirable.
Significant amounts of fibrous protein called collagen present in tissue and vessel walls make coagulation possible. Heat causes this collagen to denature, and as a result the collagen swells and becomes adhesive. For vessels, denaturing of collagen results in contraction of the vessel walls at the heated region. If heat is maintained, the vessel walls contract until the vessel is completely occluded and fluid flow stops. Various forms of energy can be used for generating heat to be applied to the vessel walls to achieve the sealing effect.
In the case of blood vessels, there are important differences between the process of blood vessel sealing as described above and tissue coagulation. Tissue coagulation involves applying energy to the tissue, without regard to the actual location of severed capillaries. Heating the tissue results in its swelling as well as the formation of a necrosis layer. This necrosis layer comprises a porous layer of desiccated cells called a spongy layer, and a gel-like compact layer formed from the denatured protein of the tissue cells and blood. The capillaries present in the tissue are small enough and have a low enough blood pressure to be effectively sealed by this process.
In contrast, larger blood vessels cannot be sealed through tissue coagulation alone, because the necrosis layer cannot hold the blood pressure of such vessels. Several types of devices known in the art may be used for sealing larger blood vessels. These devices are typically suited for the specific purpose of vessel sealing and during surgery have to be used in addition to other surgical devices.
One such device is the thermal tweezers, which incorporates a heating wire in one of two prongs and is disclosed for example in U.S. Patent Publication No. US 2006/0217706. A current passes through the wire and causes this prong to heat up. When the tweezers clamp down on a blood vessel, the heat simultaneously cuts the blood vessel and seals the two severed ends. The temperature near the wire is high enough to vaporize tissue, while the temperature further away from the wire is at a level suitable to denature the collagen in the blood vessel walls. This temperature distribution ensures that when the blood vessel is completely severed, both ends are sealed and blood no longer flows. Because of its specialized purpose, the thermal tweezers are not suitable for general tissue coagulation. Further, the device requires an exposed portion of a blood vessel onto which it can clamp. This means that the device cannot easily seal the end of a blood vessel severed during tissue dissection, where the severed end does not protrude beyond the dissection surface.
Another type of device is the electrocauterizer, which may be monopolar or bipolar. In a typical monopolar device, disclosed for example in U.S. Pat. No. 4,128,099, an active electrode is in close proximity to the treated region, such as the severed end of a blood vessel. The active electrode typically has a small surface for concentrating a relatively large amount of electrical energy. An inactive return electrode is affixed on the patient at a remote location. With this arrangement of electrodes, the electrical current passes from the active electrode to the treated region and then through the patient to the return electrode.
Among the drawbacks of a monopolar electrocautery devices is the unpredictable and uncontrollable nature of the electric current's pathway. The current may jump from one area of the surgical site to another, undesirably affecting the neighboring tissues. This, in turn, limits its utility as the device energy cannot be reliably directed to a single vessel desired to be sealed. Another drawback stems from the reliance of these devices on the electrical conductivity of liquids present at the surgical site or in a vessel to pass current. Some liquids carried in vessels, such as lymph, are nonconductive and a monopolar electrocautery device cannot be used for sealing of such vessels. Further, because the current passes through the patient's body, the current can adversely effect tissue beyond the surgical site. This essentially prohibits the use of such devices on tissues sensitive to electric currents, such as those of the heart or brain.
Bipolar electrocautery devices overcome some, but not all, of the limitations of monopolar devices. A typical bipolar device, such as the one disclosed for example in U.S. Pat. No. 7,118,570, has two electrodes in close proximity to each other, and, in operation, a current passes between them. This localizes the current to a small region, and only tissue in this region experiences the desired thermal changes. The two electrodes can be incorporated into tweezers that simultaneously apply mechanical force and thermal energy to the clamped vessel.
While more suitable for sealing vessels, a bipolar configuration also suffers drawbacks. Fundamentally, this device still applies current to the tissue, and therefore is still unsuitable for vessels carrying nonconductive liquids and tissues sensitive to electric currents. Additionally, because the vessel to be sealed has to be positioned between the two electrodes, the bipolar electrocautery devices suffer the same drawback as the thermal tweezers as it requires a part of the vessel to be exposed.
Another type of device used for sealing blood vessels is an ultrasonic device. This device utilizes an active jaw and an anvil surface for the other jaw. The active jaw uses a piezoelectric element to vibrate at ultrasonic frequencies. The vibrations caused in the tissue trapped between the two jaws create heat by internal friction. Drawbacks of this device are similar to the other devices in tweezer configurations, namely that an exposed portion of a blood vessel is required for clamping the vessel.
None of the presently known devices are capable of sealing vessels by using only heat, without electricity or mechanical force. Further, the prior devices are inconvenient for surgeons because they require a switch of devices in the middle of the procedure. For example, when a surgeon makes an incision with a scalpel, he will have to set the scalpel aside and use another device to seal vessels. This diverts the surgeon's attention and introduces another device into the surgery.
Presently, plasma devices are used for cutting, evaporating, and coagulating tissues. As such, these plasma devices can be used for sealing smaller vessels in tissue. One example is the device disclosed in U.S. Pat. No. 5,843,079. But while plasma devices are used for cutting, evaporation, and coagulation of tissues and smaller vessels under 1 mm in diameter, such devices could not be used successfully for sealing larger vessels. Because plasma devices are already used for a variety of surgery-related functions it is desirable to enable these devices to seal vessels safely and effectively.